I seen baby bed matresses for about 13 dollars at Wal-mart.
You could buy two of those and make your new cushions.
You would have to cut the matress open to get to the foam. The matress is vinyl covered.
I paid 125 dollars for a pad from a matress foam manufacturer.
I looked around and for some weird reason, foam is expensive!
A sewing machine and a futon cover work great. (Futon cover has zipper) cut to fit the matress or convert your Gaucho to a true full size bed.